1. Field of the Invention
This invention relates to a method of cleaning articles from which contaminants must be removed, and more particularly to a method of cleaning substrates for electronic devices. More specifically, this invention relates to the removal of organic films such as photoresists used when wafers for semiconductors or substrates for liquid crystal display devices are processed, and to the cleaning of wafers to remove organic contamination caused thereon over the whole wafer processing. More broadly, it also relates to the cleaning of precision metal workpieces or glass workpieces to remove their organic contamination.
2. Description of the Prior Art
To remove photoresists used in fine processing on oxide films or polysilicon films, a method is usually used in which a mixture solution of sulfuric acid (3 or 4 parts by volume) and hydrogen peroxide (1 part by volume) (which solution is called a piranha) is heated to 110 to 140xc2x0 C. and cleaning targets are immersed therein for 10 to 20 minutes. In the case when high-density ion implantation is effected via a resist mask, the resist changes in properties to become unremovable by piranha treatment, and hence ashing plasma-excited oxygen is in wide used. If, however, the whole photoresist is subjected to ashing, a trace metal due to the resist may remain on the wafer surface, and also damage which is harmful for devices may appear on the wafer surface because of high-energy plasma. Accordingly, it is common to effect ashing, leaving the resist film unremoved, followed by piranha treatment to remove the resist. In place of the hydrogen peroxide used in this piranha treatment, it has been attempted to mix ozone. However, because of a low solubility of ozone, the treatment must be made for a still longer time to remove the resist, and such a method is almost not in use.
Recently, a method of removing the resist ozone water has become available. Ozone more dissolves in water as temperature is lower. In about 5xc2x0 C. ultrapure water, ozone dissolves up to a concentration as high as 70 to 100 ppm. Where the resist is removed ozone water having such a low temperature and a high concentration, it is reported that, in the case of I-ray positive novolak resin photoresist film used widely in LSI fabrication, a film of 800 nm thick can be stripped in about 10 to 15 minutes (stripping rate: 70 to 80 nm/minute).
From atmosphere in a clean room for semiconductor device fabrication, organic matters such as dioctyl phthalate (DOP), siloxanes and hexamethyldisilazane (HMDS) contaminate the surfaces of silicone wafers, oxide films and so forth. It is known that this causes deterioration of device characteristics to lower the yield of devices.
As wet-process cleaning for removing such organic matters on silicone wafers and oxide films, the above piranha treatment has been considered to be most effective. However, SO42xe2x88x92 remains on wafers to cause fine particles under the influence of environmental atmosphere, tending to cause haze. In order to remove it completely, SC-1 treatment (standard composition: NH4OH:H2O2:H2O=1:1:5 part(s) by volume) is usually made subsequently. SC-1 treating solution has the action to decompose and remove organic matters even when made alone, and has ever been considered to be most greatly effective for the action to remove fine particles. In the SC-1 treatment, however, Fe, Al, Ca, Mg, Zn, Ni and the like in the treating solution tend to become deposited on wafers, and it is difficult to manage the cleanness of treating solutions and cleaning baths. Accordingly, it has become a conventional means for semiconductor cleaning to remove, dilute HF, chemical oxide films produced in SC-1 treatment and then make SC-2 treatment (standard composition: HCl:H2O2:H2O=1:1:6 part(s) by volume), which is considered to have a good metal-removing ability. This is called the RCA method. To remove the surface residual SO42xe2x88x92, a method is also used in which rinsing hot water in a large quantity is carried out for a long time, which, however, is inferior in the cleanness to be achievable, usually when the RCA method is made subsequently.
As cleaning methods for wafers contaminated organic matters, the treatment relying on the piranha treatment conventionally made can not be said satisfactory in view of economical advantages, productivity and safety. As a new cleaning method that can solve these problems, the method making use of ozone water has become available. This is a method in which, since ozone water having a concentration of 20 to 30 ppm is obtainable at room temperature, this oxidation power is utilized to remove organic contamination of wafers.
Higher integration of semiconductor devices, in particular, VLSI circuits, it has increasingly become important to reduce organic contamination on wafer surfaces. In roadmaps published by U.S.A. Semiconductor Industrial Society, there has been no description on surface organic carbon concentration till recently. In those published at the end of 1997, they approve a surface organic carbon concentration of 1xc3x971014 atoms/cm2, while this concentration must be made to 1.8xc3x971013 atoms/cm2 by 2009. Of course, this cleanness must be achieved also after the stripping of resist. Cleaning solutions for piranha treatment are repeatedly used in view of economical advantages. However, in an attempt that methyl silicon layers ascribable to HMDS in positive-resist primers are removed to such a high level of cleanness, it is difficult to do so for piranha cleaning solutions having deteriorated as a result of repeated use. Accordingly, the number of times for their use must be severely restricted. Hence, this results in an increase in the quantity of sulfuric acid used, bringing about not only an economical disadvantage but also a difficulty in waste-water disposal.
Since also the removal of the resist on a metal film may damage the film when treated with a strong acid, the treatment is made by dissolving the resist at about 70xc2x0 C. for about 15 minutes using N-methylpyrrolidone (NMP) as a remover. In such a case, rinsing with ultrapure water is carried out after rinsing with an organic solvent such as isopropyl alcohol. This treatment requires to use the organic solvent in a large quantity, and is undesirable in view of economical advantage and besides costly for waste-water disposal.
Accordingly, the treatment with ozone water attracts expectation. High-purity ozone water on the level of semiconductor purpose is produced by making ozone-containing high-purity gas absorbed in ultrapure water. Now, when ozone-containing gas is injected into a container holding a liquid, and where ozone concentration in the gas is represented by CG [mg/L] and ozone concentration in the liquid standing saturated with it by CL [mg/L], a partition coefficient is given as D=CL/CQ. Here, when the liquid is water, some research information gives values of D=0.2 at 25xc2x0 C., D=0.28 at 20xc2x0 C. and D=0.47 at 5xc2x0 C. Since the ozone concentration attained by means of a usual high-purity ozone gas generator is about 200 mg/L, calculation made thereon gives saturation concentrations of 40 ppm at 25xc2x0 C. and 94 ppm at 5xc2x0 C. In practical use, only concentrations a little lower than these concentrations are attained. Moreover, since ozone tends to decompose in water, the ozone concentration in an ozone water cleaning bath can not be maintained at the highest level unless ozone gas is always injected while the ozone water is circulated. Also, when there is any obstacle to flow, such as a wafer carrier, in the cleaning bath, some part on the wafer surface may come to lack in ozone to cause a decrease in resist stripping rate. Even if the resist itself can provide the stripping rate at a value of about 100 nm/minute, it takes a time twice or more the treatment time calculated from this stripping rate, in order to completely remove the resist completely up to the methyl silicon layer in respect of all wafers in the wafer carrier. More specifically, it takes 20 to 30 minutes to remove a resist film of 1 xcexcm thick.
Meanwhile, in a clean room for semiconductors, an organic matter detected on the wafer in the largest quantity is DOP in usual instances. Its quantity may often exceed 200 ng on the Six-inch wafer surface. This DOP forms a fine-spotlike oil film on the wafer surface. Contaminant fine particles having deposited thereon are strongly captured on the surface by the action of liquid cross-linking of the oil film, and can be removed with difficulty by cleaning. This phenomenon is remarkable on the wafer""s back-side surface, and in some cases such particles are in a greatly different larger quantity than those on the front-side surface. This is because, in device fabrication steps, wafers are in some cases processed in the sate the wafer backs are brought into contact with other materials such as vacuum chucks, where the surfaces of such materials are usually lipophilic and hence become contaminated with DOP or the like, and this is transferred. In semiconductor device fabrication steps, wafers are in some cases simultaneously processed in a large number in the state the back-side surface of a wafer faces the front-side surface of an adjacent wafer. In such a case, any contamination of the back with organic matters or fine particles has an adverse effect upon the device-forming surface facing thereto. Contamination with DOP or the like is known to obstruct metal contamination from being removed in the step of cleaning. The adverse effect caused by the back-side contamination also includes such metal contamination.
Under such circumstances, it is sought to solve these problems to provide an improved cleaning method.
Accordingly, an object to the present invention is to provide a method for cleaning treatment that can shorten the time taken for immersion resist stripping inclusive of complete removal of such resist primers, can also reduce the surface carbon concentration to the order of 1012 atoms/cm2 after treatment, and can be effective also as a photoresist-removing method.
Another object of the present invention is to provide a method for cleaning treatment that can powerfully remove the organic contaminants on the front- and back-side surfaces of wafers and also can remove metal contaminants.
Still another object of the present invention is to provide a cleaning method also applicable to cleaning objects other than substrates for electronic devices on account of the advantages that this removal of organic-matter contamination is very powerful and also environmental pollution can be managed with ease.
To achieve these objects, the present invention first provides a cleaner for removing contaminants, comprising an organic solvent having a partition coefficient to ozone in a gas, of 0.6 or more, and ozone having been dissolved in the solvent.
The present invention second provides a method of removing surface-deposited contaminants, comprising bringing a treating solution into contact with the surface of a treating target on which contaminants have deposited; the treating solution comprising an organic solvent having a partition coefficient to ozone in a gas, of 0.6 or more, and ozone having been dissolved in the solvent.
As a preferred embodiment of the above method, the present invention also provides a method of removing surface-deposited contaminants, comprising forming a running film (i.e., a running liquid layer in the form of a film) of the above ozone-containing treating solution on the surface of a treating target on which contaminants have deposited, and bringing the ozone-containing treating solution into contact with the surface of the treating target while replenishing the treating solution continuously or intermittently to the running film to move the solution in the form of a film.
This method is attributable to a specific organic solvent such as acetic acid in which ozone is soluble in a quantity about 10 times that in water, which is a quantity large enough even for the ozone in the running film to act on the surface-deposited contaminants. As long as the ozone concentration in the atmosphere coming into contact with the running film is higher than the equilibrium concentration with respect to ozone in the solution, the ozone readily diffuses into the running film and the ozone concentration in the solution increases up to nearly a saturation point in a short time.
Accordingly, as another preferred embodiment of the above method, the present invention still also provides a method of removing surface-deposited contaminants, comprising forming a running film of the above organic solvent on the surface of a treating target on which contaminants have deposited, and bringing the ozone-containing treating solution into contact with the surface of the treating target while replenishing the organic solvent continuously or intermittently to the running film to move the solution in the form of a film.
The present invention further provides an apparatus for removing contaminants having deposited on the surface of a treating target, the features of which will become apparent from the following description.